In photography, viewfinders are used to display the actual image formed by a lens and is a means for a photographer to aim accurately and to compose his shot.
In modern digital single lens reflex (DSLR) cameras, the image formed by the lens is projected to a mirror in front of the sensor, hinged at 45°, which reflects the image upwards to a horizontal focusing screen; this image is further projected and then reflected several times within a pentaprism before reaching the viewfinder eyepiece; the pentaprism is in place so that the image seen through the eyepiece is right way up and right way laterally. When the shutter release is pressed, the mirror flips out of the way almost instantaneously to expose the sensor to the image formed by the lens, during this time the display in the viewfinder is not available. DSLRs also feature a monitor for reviewing taken pictures, displaying and adjusting settings.
Recently, another form of viewfinder, the electronic viewfinder or EVF has become increasing popular, mainly due to the reduction in size of the camera; an EVF displays the image directly captured by electronically by the sensor, eliminating the need for the mirror and the pentaprism. Furthermore, unlike optical viewfinders in DSLR cameras, the EVF can also display previously taken pictures for review.
Usually, cameras that employ the EVF also have monitors; sometimes the monitor can be used for aiming and composing, since the display is larger and it allows the photographer to be further away from the camera while composing; this is necessary when shooting at special angles, such as from above, below or when it is impossible for the photographer to get close to the object. The monitor is also useful when shooting videos with the camera, as it is desirable for the photographer to be able to look at the whole surrounding environment with his eyes rather than merely looking at the image through the viewfinder.
On the other hand, the EVF is sometimes preferred because the photographer can bring the camera closer to his body; this is a more stable position minimizing vibration of the camera and blurred images. The EVF is also necessary when shooting in bright light as it is very difficult to see the display in the monitor; the EVF also supports dioptric adjustments for short or long sighted photographers.
In order to save battery power, these cameras are often designed so that either the EVF or the monitor is on at any instance and an electromagnetic proximity sensor is used so that when an object, such as a human eye, gets close to the EVF, it will automatically switch on while the monitor switches off at the same time.
In underwater photography, a housing is used to protect the camera from water ingress, it also contains means for the photographer to access some or all controls of the camera placed within the housing.
When a digital camera equipped with a proximity sensor for switching between the EVF and monitor is placed inside a housing, a problem arises since the volume surrounding the camera inside the housing is often relatively small so that the housing itself can cause activation of the proximity sensor which turns the EVF on and the monitor off. This means that photographers cannot use the monitor for composing, reviewing and adjusting settings, which is extremely undesirable for underwater usage.
Some cameras allow the photographer to choose which of the EVF or the monitor he would like to turn on. However, the photographer has to go into menu of the camera to make the selection and to switch between the two; the amount of time that its takes to make the selection varies from camera to camera but it could take as long as a couple of minutes. Other cameras do not support such a function, meaning that the EVF is permanently on and the monitor is permanently off when the camera is placed inside a housing.
This is disadvantageous and limits the effectiveness of a camera when used in an underwater camera housing.